Lamella settler crystallizer

ABSTRACT

A crystallizer which incorporates a lamella settler and which is particularly applicable for use in batteries and power cells for electric vehicles or stationary applications. The lamella settler can be utilized for coarse particle separation or for agglomeration, and is particularly applicable to aluminum-air batteries or power cells for solving the hydrargillite (aluminum-hydroxide) removal problems from such batteries. This invention provides the advantages of very low energy consumption, turbulence, shear, cost and maintenance. Thus, due to the low shear and low turbulence of this invention, it is particularly effective in the control of aluminum hydroxide particle size distribution in the various sections of an aluminum-air system, as well as in other electrochemical systems requiring separation for phases of different densities.

The invention described herein arose in the course of, or under,Contract No. W-7405-ENG-48 awarded the University of California by theU.S. Department of Energy.

This is a Division of U.S. patent application No. 931,825 filed Nov. 18,1986, now U.S. patent application No. 4,735,872 issued Apr. 5, 1988.

BACKGROUND OF THE INVENTION

This invention pertains to the application of lamella (or tube) settlersto crystallizer design in general; in particular, application ofsettlers to compact crystallizers such as used in advanced typebatteries and power cells for electric vehicle or stationaryapplications, and more particularly to a new approach for solving thehydrargillite removal problem from aluminum-air batteries.

The beneficial effect on settling of suspensions by having inclinedrather than vertical tubes is well known. In current practice, theinclined plate settlers, commonly known as lamella settlers, consist ofparallel plates, usually arranged in a stacked array, which formchannels into which a slurry is fed. Under the action of gravity, thesolids deposit on the upper surface of the inclined plate at the bottomof each channel and flow to the dense slurry collector at the bottom ofthe settler. The clear liquid rides under the lower surface of the plateat the top of each channel and is collected at the top of the lamellasettler.

Lamella settlers are very effective, low energy intensity sedimentationdevices which are being extensively used in water treatment plants andsome applications in the mining and minerals industry. Lamella settlerscan be used in place of other commonly used separation devices, such ashydrocyclones, elutriators, thickeners, etc. While lamella settlers donot provide as sharp a separation of fine and coarse particles ashydrocyclones, when used as clarifiers they have a number of advantagesover hydrocyclones, such as low energy consumption and low shear.

Lamella settlers are exemplified by U.S. Pat. No. 4,151,084 issued inApr. 1979 to R. F. Probstein et al, as well as by numerous publications,such as "Lamella and Tube Settlers. 1. Model and Operation", W. Leung etal, Ind. Eng. Chem. Process Dev., 22, 58-67 (1983), and "TheSedimentation of Polydisperse Suspensions In Vessels Having InclinedWalls", R. H. Davis et al, Int. J. Multiphase Flow, 8(6), 571-585(1982).

Crystallization is a well known technology and various types ofcrystallizers are known in the art. However, one of the aspects ofcrystallization, particularly when applied to small, compact systems,such as required for successful application of advanced batteries/powercells to electric vehicles, is the control of the particle sizedistribution of the crystals. Very fine particles are continuously beingformed by breakage of larger crystals and by secondary nucleation; bothof these are promoted by high shear conditions.

The control of fines is a general problem in industrial applications ofcrystallization. In industrial applications the control of fines isusually performed on a side stream containing only fine particlessuspended in the mother liquor; the suspension is heated above thetemperature required for complete solids dissolution, cooled andreturned to the crystallizer. However, this means of finescontrol/removal is of limited applicability to compact, mobile systems;in the case of the aluminum-air power cell the solutions are highlysupersaturated and heating to 150°-230° C. would be required for finesremoval. The equipment required to obtain such heating and holding thesuspension at elevated temperature and pressure would add a prohibitivepenalty to the weight, volume and cost of such a power cell system.

Another way to remove the excess fine crystals is by agglomeration, aprocess where small crystals coalesce and are bonded together to form alarger agglomerate. Experimental work on agglomeration in suspension haspointed out the necessity for low shear in the agglomerating process;the initial agglomerates are very fragile and break-up very easily.

A problem arising in the design of electrochemical reactors involvingsuspended solids (or two liquid phases of different densities) is thatthere is a need to:

1. Maintain a low solids (or second phase) concentration in the cells.

2. Obtain efficient contacting between the two phases (solid-liquid orliquid-liquid) in a separate tank.

3. Minimize energy consumption in all auxiliary devices.

Thus, separation devices of some sort are required. An additionalproblem in the aluminum-air and lithium-air battery is the removal ofthe reaction product from the system.

The prior development efforts of the aluminum-air power cell, forexample, utilized hydrocyclones for the removal of most of the solidsfrom the electrolyte flow returning to the cell stack and for coarseproduct removal. Such an arrangement utilizing cyclone separators isdescribed and illustrated in document UCID-20356 entitled "Aluminum-AirPower Cell Research and Development Annual Report Summary, CY 1984", A.Maimoni, Feb. 27, 1985. However, as recognized in the art, cyclone typesystems involve high shear effects and involve relatively high energyconsumption, with the high shear adversely affecting crystal growth andagglomeration.

Thus, there is a need for an efficient, low shear, low energyconsumption particle separation process. In addition, in aluminum-airbatteries/power cells there is a need for an efficient and effectivemeans for coarse product separation and removal, and to provide forcrystal growth and agglomeration.

Therefore, it is an object of this invention to provide an apparatus forparticle separation/crystallization which utilizes low shear and lowenergy consumption.

A further object of the invention is to provide a particlecrystallization means which incorporates a lamella settler.

A further object of the invention is to provide a new approach forsolving the hydrargillite removal problem from aluminum-air batteries.

A still further object of the invention is to provide an apparatus whichcombines lamella settlers and crystallizers for use in compact batteriesand power cells for improved removal of reaction products.

Another object of the invention is to provide a lamella settlercrystallizer, particularly adapted for use in electrochemical systemsrequiring separation of phases of different densities.

Another object of the invention is to provide an improved crystallizerfor separation of solid reaction products from liquids whichincorporates a lamella settler.

Another object of the invention is to provide a lamella settlercrystallizer which operates in a low shear environment, minimizesbreaking of crystals by abrasion, and is essential to the formation ofagglomerates, while operating at low pumping costs.

Still another object of the invention is to provide a lamella settlercrystallizer which is particularly adapted for aluminum-airbatteries/power cells, and includes a mechanism for coarse productseparation and removal while maintaining the coarse fraction of thecrystal population within the crystallizer, and providing a dilutesuspension of fine particles for return to the cell while retaining theparticles in the agglomerating range in a low shear environment.

Other objects and advantages of the invention will become apparent tothose skilled in the art from the following description and accompanyingdrawings.

SUMMARY OF THE INVENTION

The above objects of the invention are carried out by an apparatus, forcarrying out a particle separation process, which utilizes a lamellasettler and which operates at low shear with low energy consumption. Theinvention, which may be utilized in various particle separationapplications, is particularly applicable to advanced type batteries andpower cells for electric vehicles, etc., such as the aluminum-air,zinc/Redox, and lithium-air batteries. These types of batteries/powercells require crystallization of the reaction products for their normaloperation, and thus compact crystallizers, such as provided by thisinvention, is essential to effective operation, due to its low shear andlow energy consumption. The apparatus of this invention enables coarseparticle separation and agglomeration.

The lamella settler crystallizer of this invention incorporates at leastone lamella settler which is inclined at about 60° to the horizontal,and basically consists of a set of parallel plates or parallel tubesinto which a slurry is fed to obtain gravitational separation, with theclarified liquid collecting under each of the plates and flows upward tothe overflow, the solids settle and flow along the bottom of each plateto be collected. In a crystallization context, lamella settlers producelow shear and low energy consumption, and the volume required for thelamella settler does not penalize the system, because the settler is anintegral part of the crystallizer. Thus, crystal growth andagglomeration take place within the settler.

In an aluminum-air power cell, for example, the lamella settlercrystallizer is connected to the electrolyte storage tank and has twodistinct uses: (1) as a means of coarse product separation and removal,and (2) as a means for maintaining the coarse fraction of the crystalpopulation within the crystallizer, providing a dilute suspension offine particles for return to the storage tank of the cell and providinga mechanism for retaining the particles in the agglomerating range (0.5to 30 micrometers) in a low shear environment. The crystallizer of thisinvention may include a pair of settlers connected in series, with thelater settler being connected to a coarse product conveyor and storagearrangement, while the fine particles are returned to the electrolytestorage tank. Thus, the invention provides a new approach for solvingthe hydrargillite removal problem form aluminum-air batteries/powercells, for example, although it is applicable to other power cells orbatteries requiring similar solids removal, as well as othercrystallization and particle separation applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a lamella settler illustrating the flow pathstherethrough;

FIG. 2 schematically illustrates an embodiment of the invention forcoarse particle separation; and

FIG. 3 illustrates an aluminum-air power cell incorporating theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a lamella settler crystallizer forcrystallization and particle separation. The invention is particularlyuseful for coarse product removal and as a low shearagglomeration/separation device. The invention has particularapplication to systems which require crystallization of the reactionproducts for their normal operation, such as advanced batteries/powercells, exemplified by the aluminum-air, zinc/Redox, and lithium-airbatteries.

Due to the low shear, low energy consumption of the lamella settlercrystallizer of this invention it is particularly applicable toaluminum-air batteries, and provides: (1) a means of coarse productseparation and removal, and (2) a means for maintaining the coarsefraction of the crystal population within the crystallizer, providing adilute suspension of fine particles for return to the cell and providinga mechanism for retaining the particles in the agglomerating range (0.5to 30 micrometers). This is particularly beneficial in the agglomerationof hydrargillite (aluminum-hydroxide) particles.

Where separation afforded by gravity alone, using an inclined lamellasettler crystallizer, is not enough, a vertical or near vertical lamellasettler section may be rotated around an axis (such as a cylindricalsection rotating around a vertical axis), and differs from aconventional centrifuge due to a much lower rotation speed.

While the following description of the lamella settler crystallizer ofthis invention will be directed to its application to aluminum-air powercells/batteries, such is not intended to limit it to this application.The invention can be utilized in other types of power cells/batteries orelectrochemical systems requiring similar solids removal, as well as inother types of crystallization and particle separation applications. Theinvention is particularly applicable to particle separation systemswhere low shear and/or low energy consumption are required for efficientoperation of the separation process.

A. Lamella Settlers:

As pointed out above, the beneficial effects of settling of suspensionson an inclined lamella settler, such as illustrated in FIG. 1 are wellknown. In current practice the lamella settler utilizes inclinedparallel plates or parallel tubes, arranged as a stacked array, whichform channels into which slurry is fed. Under the action of gravity thesolids deposit on the upper surface of the inclined plates at the bottomof each channel and flow to the dense slurry collector at the bottom(see FIG. 1). The clear liquid rides under the lower surface of theplates at the top of each channel and is collected at the top of thelamella settler. Since lamella settlers, and their construction andoperation, are well known, a detailed description and illustration ofsuch is not deemed necessary for an understanding of the basicoperational characteristics of a lamella settler in this invention.

The flow rate of clarified product from the lamella settler of FIG. 1 isapproximately given by:

    S(t)=(v.sub.o b/cos α)[1+(H/b)sin α]

where:

S(t)=volumeric rate of production of clarified fluid per unit depth inthe third dimension of the vessel; cm³ /sec/cm,

v_(o) =vertical settling rate of the particles (i.e., the settling ratein a vertical vessel); cm/sec,

α=angle of inclination of the plates with respect to the vertical,

b=spacing between plates; cm,

H=vertical height of the suspension; cm.

The concentration of solids present in the clarified product is afunction of the particle size distribution of solids in the feed and ofthe flow rate of clarified products. It has also been found that lamellasettlers provide advantages in the context of crystallization:

(a) low shear environment, which minimizes breaking of crystals byabrasion and is essential to the formation of agglomerates;

(b) low pumping costs, since the main contributor to the pressure dropis the hydrostatic head of the fluid being pumped;

(c) relative to vertical settling, provide large settling enhancementfactors (about 25/1) and low volume and weight requirements; and

(d) simplicity and low cost.

However, lamella settlers do require more volume than energy intensivedevices, such as hydrocyclones, centrifuges, etc.

B. Crystallization:

One of the key aspects of crystallization, particularly when applied tosmall, compact systems, such as required for successful application ofadvanced batteries/power cells to electric vehicles, is the control ofthe particle size distribution of crystals. Very fine particles arecontinuously being formed by breakage of larger crystals and bynucleation. Crystal breakage is promoted by existence of high shearconditions; in the case of the aluminum-air systems, it has been foundthat the shear produced by centrifugal pumps, hydrocyclones used forfines separation; etc., lead to breakage of large particles and inhibitthe agglomeration process. In the aluminum-air power cell the finecrystals have to grow to a minimum size (at least 20 micrometer) beforeremoval from the system; in others, such as the zinc/Redox system,control of the population of very fine crystals is required to maintainthe desired properties in the slurry circulating to the electrodes.

The control of fines, as pointed out above, is a general problem inindustrial applications of crystallization. In industrial applicationsthe control of fines is usually performed on a side stream containingonly fine particles suspended in the mother liquour; the suspension isheated above the temperature required for complete solids dissolution,cooled and returned to the crystallizer. However, this means of finescontrol/removal is of limited applicability to compact, mobile systems;in the case of the aluminum-air power cell the solutions are highlysupersaturated and heating to 150°-230° C. would be required for finesremoval. The equipment required to obtain such heating and holding thesuspension at elevated temperature and pressure would add a prohibitivepenalty to the weight, volume and cost of the system.

As pointed out above, another way to remove the excess fine crystals isby agglomeration, a process where small crystals coalesce and are bondedtogether to form a larger agglomerate. Experimental work onagglomeration in suspension has pointed out the necessity for low shearin the agglomerating process; the initial agglomerates are very fragileand break-up very easily.

C. Application of Lamella Settlers to Crystallization:

The above sections (A and B) summarized the main processes operating incrystallization, operation of lamella settlers, and the reasons forneeding efficient, low shear, low energy consumption particle separationprocess. In a crystallization context, the volume required for thelamella settler does not necessarily penalize the system, because thesettler is an integral part of the crystallizer; crystal growth andagglomeration take place within the settler.

As applied to the aluminum-air system, there are two distinct uses forlamella settlers: (a) as a means of coarse product separation andremoval, (b) as a means for maintaining the coarse fraction of thecrystal population within the crystallizer, providing a dilutesuspension of fine articles for return to the cell and providing amechanism for retaining the particles in the agglomerating range (0.5 to30 micrometers) in a low shear environment.

Examples of these applications are given below:

(a) Coarse particle separation:

An apparatus embodiment is shown schematically in FIG. 2, but it shouldnot be construed as the only means of utilizing a lamella settler forcoarse product removal. A pipe 10 at the bottom 11 of a crystallizer, C,is connected to a lower end 12 of a lamella settler, S. A slurrycontaining medium-sized particles flows to the top 13 of the settler S,where a pump, P, via pipes 14 and 15, returns it to the crystallizer C.Coarse product crystals flow down on a lower surface of end 12 of thesettler S to a collection chamber or tube 16 which feeds a screwconveyor, SC, for example. Wash water indicated at 17 can be added at anintermediate location in the screw conveyor SC and the washed productpasses into pipe 18 and is collected in a product container, F, with thewash water passing through tube 16 and into settler S for replenishingwater used in the system. The effectiveness of washing of the product inconveyor SC and the tube 16 can be increased by using a zig-zagarrangement in tube 16 (similar to that of FIG. 3); the wash waterflowing upwards through the falling particles at each corner of thezig-zag carries the fine particles upwards, thus producing a bottomproduct in which the coarse particles are more concentrated. Preliminarycalculations indicate that a lamella settler (see FIG. 1) with

H=60 cm

b=1 cm

α=50°

can separate 50 micrometer particles of aluminum hydroxide(hydrargillite) from supersaturated sodium aluminate solutions(density≃1.2 G/cm³, viscosity≃1.8 centipoise) containing 25 vol % solidsat

S=2.2 cm³ /sec/cm

Industrial crystallizers often use an elutriation section at the bottomof the crystallizer; the lamella settler provides similar separation ofthe final product in a smaller volume.

(b) Agglomeration:

The application of lamella settlers to agglomeration will be illustratedin the context of the aluminum-air power cell system (see FIG. 3).Experimental work verifying this invention has demonstrated thatparticles smaller than 30 micrometer will agglomerate into the 30micrometer range; the system also requires that the particleconcentration in the electrolyte returning to the cell from thecrystallizer be kept as low as practicable. The lamellasettler/agglomerating section can be designed to retain particles in therange 10 to 30 micrometers; it is estimated that for the settlerdimensions and suspension indicated above, a clarification rate of 0.15cm³ /sec/cm can be obtained.

FIG. 3, incorporates the invention into an aluminum-air powercell/battery system. The overall system of FIG. 3 is generally similarto the aluminum-air power cell illustrated and described inabove-referenced report UCID-20356, except that the crystallizer andcyclones of the prior cell have been replaced by a lamella crystallizerarrangement in accordance with the present invention. The FIG. 3embodiment incorporating the invention provides for the separation ofthe aluminum hydroxide (hydrargillite) reaction products from theelectrolyte of aluminum-air batteries. In contrast to other compactparticle separation devices, such as filters and hydrocylones, used forexample in the power cell of above-referenced report UCID-20356, lamellasettler crystallizers provide the following unique advantages: (1) verylow energy consumption, (2) very low turbulence, (3) very low shear, and(4) very low cost and maintenance requirements. The low shear and lowturbulence are essential to control the aluminum hydroxide particle sizedistribution in the various sections of the aluminum-air system. Lowshear and low turbulence are required in aluminum-air systems, forexample, to: (1) control the number of new fine crystals produced bysecondary nucleation, and (2) agglomerate the fine particles intocoarser aggregates. Agglomeration of the fine particles is essential toobtain small volume and small weight aluminum-air systems.

Referring now to FIG. 3, the aluminum-air power cell system illustratedtherein comprises a cell stack 20, which may be of the type illustratedin FIG. 1 of above-referenced report UCID-20356, connected at one sideto a heat exchanger 21 via an H₂ recombiner 22 and a CO₂ scrubber 23through lines or pipes 24-24' and 25-25'. Cell stack 20 is connected atthe opposite side to an electrolyte storage tank 26 via a pair of pumps27 and 28 and interconnecting lines or pipes 29, 30, 31 and 32, with aheat exchanger 33 connected between line 32 and cell stack 20. An airpump 34 is connected to pump air through heat exchanger 21 and scrubber23 to cell stack 20 via lines 25'-25, the air being discharged from cellstack 20 through recombiner 22 and heat exchanger 21 via lines 24-24' toatmosphere, as indicated by flow arrows, or for recirculation throughthe cell. Electrolyte from the lower end of storage tank 26 is pumpedthrough line 31 by pump 27 into pump 28 and through line 32 and heatexchanger 33 into cell stack 20 for reaction in the cell stack as knownin the art, after which the electrolyte discharges from cell stack 20via line 30 which is connected to each of pump 28 and storage tank 26.Lines 31 and 30 meet at a junction 35 connected to the inlet of pump 28.A valve may be located at junction 35 to allow draining of electrolytefrom the cell stack 20 during shut-down via lines 30 and 31.

An inclined lamella settler crystallizer 36 is connected to electrolytestorage tank 26 at the bottom or lower end thereof by pipes or lines37-37' and at the top or upper end thereof by a pipe or line 38, with apump 39 mounted between lines 37 and 37'. An inlet of pump 27 is alsoconnected to line 37. Electrolyte discharged from cell stack 20 intostorage tank 26 is then pumped via pump 39 through inclined lamellasettler crystallizer 36, as indicated by flow arrows, wherein largerparticles in the electrolyte are removed and/or the finer particles areagglomerated as described above. The larger particles under the actionof gravity are collected in the lower end or collector section 40 ofcrystallizer 36, and the fine particles which remain in the electrolytepass through the crystallizer and return to storage tank 26 via line 38.The small particles within the crystallizer 36 are allowed toagglomerate within the lamella settler arrangement due to the low shearenvironment of the crystallizer 36 as discussed above. In thisembodiment, the lamella settler crystallizer 36 is positioned at anincline of 60° with respect to horizontal.

The larger particles of hydrargillite which settle into the collector orlower end 40 of crystallizer 36 are pumped with electrolyte, asindicated by flow arrows, to an inclined lamella settler 41 which isconnected to crystallizer 36 via lines or pipes 42 and 42' between whichis connected a pump 43, the electrolyte passing through settler 42 andis directed back into crystallizer 36 via a line or pipe 44 and at apoint located above the lower end or collector section 40 of thecrystallizer 36. Lamella settler 41, like crystallizer 36 is inclined ata 60° angle with respect to the horizontal. Hydrargillite is collectedin lower end or collector section 45 of settler 41 and drops via azig-zag pipe or tube 45' into a conveyor mechanism 46 as indicated byarrow. Due to the zig-zag of pipe or tube 45' this results in acollection of coarse or larger particles at the bottom of each sectionof the zig-zag with the fine particles being on the upper portion ofeach section. Wash water from a storage tank 47 passes through a line orpipe 48 onto conveyor mechanism 46, as indicated by flow arrow, and thenpasses up zig-zag tube 45' into settler 41 for replenishing the water inthe system. After being washed, the hydrargillite (Al(OH)₃) is collectedin a storage tank or container 49 as indicated by arrow 50. Zig-zag pipe45' enhances the effectiveness of the wash/product separating operation.Thus, the electrolyte discharged from the cell stack 20 is circulatedthrough lamella settler crystallizer 36 and lamella settler 41, whereinthe larger particles or reaction products are removed from theelectrolyte, and the small or fine particles are allowed to crystallizein the low shear environment of the crystallizer 36.

The electrochemical system of FIG. 3 may include a reaction chamberassociated with electrolyte storage tank 26 (the reaction being in thecirculation of the electrolyte through the system), and the tank 26 maybe broadly considered as a storage/reaction chamber.

It has thus been shown that the invention provides an improvedcrystallizer and particle separation apparatus, as well as a newapproach for solving the hydrargillite (aluminum hydroxide) removalproblem from aluminum-air batteries. Again, while the invention has beenillustrated and described relative to aluminum-air systems, theinvention can be used in other electrochemical systems requiringseparation of phases of different densities, such as lithium hydroxideof the lithium-air battery, the zinc ferricyanide and some versions ofthe zinc-air battery, as well as other crystallization and particleseparation applications.

While embodiments incorporating the invention have been illustrated anddescribed, modifications and changes will become apparent to thoseskilled in the art, and it is intended to cover in the appended claimsall such modification and changes as come with the scope of the appendedclaims.

What is claimed is:
 1. In an apparatus requiring separation of particlesfrom a fluid circulating through the apparatus, and which particles aregenerated in the apparatus, the improvement comprising:a lamella settlercrystallizer which functions as a coarse product separation and removalmeans and as a low shear agglomeration/separation means, saidcrystallizer being connected at a lower end thereof to the apparatus toreceive at least a portion of the fluid containing particles so as toseparate particles above a predetermined size from the fluid as thefluid passes along a length thereof, and said crystallizer beingconnected at an upper end thereof to the apparatus to return the fluidcontaining particles below a predetermined size to be circulated throughthe apparatus, said crystallizer being positioned such that alongitudinal axis thereof is at an angle with respect to horizontal;means for directing the fluid through said crystallizer; and meansconnected to said lower end of said crystallizer for removing therefromparticles which have been separated from the fluid.
 2. The improvementof claim 1, wherein said means for removing particles from saidcrystallizer includes a conveyor mechanism operatively connected to saidcrystalizer and a collection means connected to said conveyor mechanism.3. The improvement of claim 2, additionally including means for washingparticles as they move along said conveyor mechanism.
 4. The improvementof claim 1, wherein said means for removing particles from saidcrystallizer includes a lamella settler operatively connected to saidcrystallizer and positioned at an angle with respect to horizontal, andmeans for directing at least the particles along at least a portion of alongitudinal length of said settler.
 5. The improvement of claim 4,wherein said means for removing particles additionally includes aconveyor mechanism operatively connected to said lamella settler.
 6. Theimprovement of claim 4, additionally including means for washingparticles as they move along said conveyor mechanism.
 7. The improvementof claim 4, wherein said crystallizer and said lamella settler arepositioned such that said angle is about 60°.
 8. The improvement ofclaim 4, wherein the particles separated from the fluid in saidcrystallizer have a size of at least 0.5 micrometers.
 9. The improvementof claim 4, wherein the particles separated from the fluid in saidcrystallizer for low shear agglomeration are of a size in the range of0.5 to 30 micrometers.
 10. The improvement of claim 1, wherein saidcrystallizer is positioned such that said angle is about 60°.
 11. Theimprovement of claim 1, wherein the particles separated from the fluidin said crystallizer for low shear agglomeration are of a size having arange of 0.5 to 30 micrometers, said crystallizer being constructed toretain particles in the agglomerating range of 0.5 to 30 micrometers soas to function as a low shear agglomeration means for the separatedparticles.
 12. The improvement of claim 1, wherein the particlesseparated from the fluid in said crystallizer have a size in the rangeof about 0.5 to about 50 micrometers.
 13. The improvement of claim 1,wherein said lamella settler crystallizer is constructed and arranged toprovide: means for coarse particle separation and removal, means formaintaining a coarse fraction of a crystal population within thecrystallizer, means for providing a dilute suspension of fine particlesfor return to the apparatus, and means for retaining particles withinthe crystallizer for agglomeration thereof.
 14. The improvement of claim1, wherein the coarse particles separated from the fluid have a sizegreater than about 30 micrometers, wherein the fine particles retainedin the fluid have a size less than about 0.5 micrometers, and whereinthe particles retained for agglomeration have a size in the range ofabout 0.5 to about 30 micrometers.